You have to remember that the faster a browser gets, the harder it is to keep improving. V8 is getting closer and closer to running at native speed, and every millisecond they improve is an accomplishment. Meanwhile, Firefox is adding a whole new compiler they didn't have before, so massive improvements are to be expected. They're also cherry-picking code from both V8 and Webkit to improve their performance where it makes sense. For example, they ported over YARR (Webkit's regular expression engine) for FF4.

That said, there's no way FF4 is going to be faster than Chrome when it comes out. It should be about on par with Webkit, which is plenty competitive. V8 will maintain a lead.